TALLAHASSEE, Fla. — On the morning of the primary here in August, the local elections board met to decide which absentee ballots to count. It was not an easy job.

The board tossed out some ballots because they arrived without the signature required on the outside of the return envelope. It rejected one that said “see inside” where the signature should have been. And it debated what to do with ballots in which the signature on the envelope did not quite match the one in the county’s files.

“This’r’ is not like that ‘r,’” Judge Augustus D. Aikens Jr. said, suggesting that a ballot should be rejected.

Ion Sancho, the elections supervisor here, disagreed.

“This ‘k’ is like that ‘k,’” he replied, and he persuaded his colleagues to count the vote.

Scenes like this will play out in many elections next month, because Florida and other states are swiftly moving from voting at a polling place toward voting by mail.

In the last general election in Florida, in 2010, 23 percent of voters cast absentee ballots, up from 15 percent in the midterm election four years before. Nationwide, the use of absentee ballots and other forms of voting by mail has more than tripled since 1980 and now accounts for almost 20 percent of all votes.

Yet votes cast by mail are less likely to be counted, more likely to be compromised and more likely to be contested than those cast in a voting booth, statistics show. Election officials reject almost 2 percent of ballots cast by mail, double the rate for in-person voting.

“The more people you force to vote by mail,” Sancho said, “the more invalid ballots you will generate.”

Election experts say the challenges created by mailed ballots could well affect outcomes this fall and beyond. If the contests next month are close enough to be within what election lawyers call the margin of litigation, the grounds on which they will be fought will not be hanging chads but ballots cast away from the voting booth.

In 2008, 18 percent of the votes in the nine states likely to decide this year’s presidential election were cast by mail. That number will almost certainly rise this year, and voters in two-thirds of the states have already begun casting absentee ballots. In four Western states, voting by mail is the exclusive or dominant way to cast a ballot.

The trend will probably result in more uncounted votes, and it increases the potential for fraud. While fraud in voting by mail is far less common than innocent errors, it is vastly more prevalent than the in-person voting fraud that has attracted far more attention, election administrators say.

In Florida, absentee-ballot scandals seem to arrive like clockwork around election time. Before this year’s primary, for example, a woman in Hialeah was charged with forging an elderly voter’s signature, a felony, and possessing 31 completed absentee ballots, 29 more than allowed under a local law.

The flaws of absentee voting raise questions about the most elementary promises of democracy.

“The right to have one’s vote counted is as important as the act of voting itself,” Justice Paul H. Anderson of the Minnesota Supreme Court wrote while considering disputed absentee ballots in the close 2008 Senate election between Al Franken and Norm Coleman.

Voting by mail is now common enough and problematic enough that election experts say there have been multiple elections in which no one can say with confidence which candidate was the deserved winner. The list includes the 2000 presidential election, in which problems with absentee ballots in Florida were a little-noticed footnote to other issues.

In the last presidential election, 35.5 million voters requested absentee ballots, but only 27.9 million absentee votes were counted, according to a study by Charles Stewart III, a political scientist at the Massachusetts Institute of Technology. He calculated that 3.9 million ballots requested by voters never reached them; that another 2.9 million ballots received by voters did not make it back to election officials; and that election officials rejected 800,000 ballots. That suggests an overall failure rate of as much as 21 percent.

Some voters presumably decided not to vote after receiving ballots, but Stewart said many others most likely tried to vote and were thwarted.

“If 20 percent, or even 10 percent, of voters who stood in line on Election Day were turned away,” he wrote in the study, published in The Journal of Legislation and Public Policy, “there would be national outrage.”

The list of very close elections includes the 2008 Senate race in Minnesota, in which Franken’s victory over Coleman, the Republican incumbent, helped give Democrats the 60 votes in the Senate needed to pass President Barack Obama’s health care bill. Franken won by 312 votes, while state officials rejected 12,000 absentee ballots. Recent primary elections in New York involving Republican state senators who had voted to allow same-sex marriage also hinged on absentee ballots.

There are, of course, significant advantages to voting by mail. It makes life easier for the harried, the disabled and the elderly. It is cheaper to administer, makes for shorter lines on election days and allows voters more time to think about ballots that list many races. By mailing ballots, those away from home can vote. Its availability may also increase turnout in local elections, though it does not seem to have had much impact on turnout in federal ones.

Still, voting in person is more reliable, particularly since election administrators made improvements to voting equipment after the 2000 presidential election.

In Leon County, Fla., the rejection rate for absentee ballots is less than 1 percent. The instructions Sancho provides to voters are clear, and the outer envelope is a model of graphic design, with a large signature box at its center.

The envelope requires only standard postage, and Sancho has made arrangements with the post office to pay for ballots that arrive without stamps.

Still, he would prefer that voters visit a polling place on Election Day or beforehand so that errors and misunderstandings can be corrected and the potential for fraud minimized.

“If you vote by mail, where is that coming from?” he asked. “Is there intimidation going on?”

Election law experts say that pulling off in-person voter fraud on a scale large enough to swing an election, with scores if not hundreds of people committing a felony in public by pretending to be someone else, is hard to imagine, to say nothing of exceptionally risky.

There are much simpler and more effective alternatives to commit fraud on such a scale, said Heather Gerken, a law professor at Yale.

“You could steal some absentee ballots or stuff a ballot box or bribe an election administrator or fiddle with an electronic voting machine,” she said. That explains, she said, “why all the evidence of stolen elections involves absentee ballots and the like.”